The invention relates generally to x-ray tubes, and more particularly to structures and methods of assembly for the spiral groove bearing (SGB) utilized in an x-ray tube.
X-ray systems may include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, may be located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. The object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. In many configurations, the anode structure is supported by a liquid metal bearing structure, e.g., a spiral groove bearing (SGB) structure, formed with a support shaft disposed within a sleeve or shell to which the anode is attached and that rotates around the support shaft. The spiral groove bearing structure also includes spiral or helical grooves on various surfaces of the sleeve or shell that serve to take up the radial and axial forces acting on the sleeve as it rotates around the support shaft.
Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into an axle formed at least partially of the sleeve that supports the anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. The x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the shaft supporting the target.
Advantages of liquid metal bearings such as spiral groove bearings in x-ray tubes include a high load capability and a high heat transfer capability due to an increased amount of contact area. Other advantages include low acoustic noise operation as is commonly understood in the art. Gallium, indium, or tin alloys are typically used as the liquid metal in the bearing structure, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube. However, liquid metals tend to be highly reactive and corrosive. Thus, a base metal that is resistant to such corrosion is desirable for the components that come into contact with the liquid metal bearing, such as the shaft of the anode assembly that is rotated for the purpose of distributing the heat generated at a focal spot.
As a result, the structure of the sleeve to which the anode is connected and the support shaft must be capable of withstanding the high temperatures and mechanical stresses created within the x-ray tube, as well as be able to withstand the corrosive effects of the liquid metal bearing. In prior art bearing constructions, a refractory metal such as molybdenum or tungsten can be used as the base material for the construction of the sleeve or shell as well as for the other bearing components. Not only are such materials resistant to corrosion and high temperatures, but they tend to be vacuum-compatible and thus lend themselves to an x-ray tube application. In addition, cooling of the bearing structure can be effected by flowing a cooling fluid into the center of the support shaft to thermally contact the heat taken from the anode by the sleeve and liquid metal bearing fluid.
However, these materials have a low weldability and are difficult to machine, such that bearing components of these materials are hard to manufacture without surface imperfections that enable leaks to occur in the seals. Also, due to the low galling/wear properties of the refractory materials, these surface imperfections, even if not present after machining, can occur during normal use of the tube resulting in the formation of fluid leaks, thereby shortening the useful life of the tube.
In an alternative construction for a liquid metal/spiral groove bearing structure, other metals, such as steel, can be utilized in place of the refractory metals for the construction of the sleeve and support shaft. While steel has a lower resistance to corrosion by the liquid metal fluid, it also has the benefits of low cost compared to the refractory metals, good machinability, good galling/wear characteristics, and good weldability. As such, these metals, e.g., steel, can be more easily constructed and joined to form the bearing sleeve.
However, one significant drawback to steel is its lower thermal conductance and higher thermal growth, which can limit bearing life by causing deformation in the bearing components formed of the steel, and a consequent alteration in the size of the gap formed between the rotating and stationary components of the bearing assembly, leading to metal to metal contact, i.e., wear, that reduces the useful life of the bearing assembly and associated x-ray tube.
In one prior art attempt to address this issue, as disclosed in US Patent Application Publication No. 2011/0280376, in which the stationary component of a bearing assembly is formed with various structures to maintain the gap size between the stationary component (the shaft) and the rotating component (the sleeve). The structures included within the shaft include inserts having different thermal expansion characteristics from the remainder of the material forming the shaft where the inserts can expand to maintain the size of the gap, a mechanical or hydraulic piston operable to expand the shaft to maintain the size of the gap, and structures within the shaft that draw heat toward multiple spots on the sleeve and the shaft to lessen the amount of deformation of the sleeve due to the heating of the sleeve during operation.
However, in each of these embodiments of the prior art solution, the structures increase the complexity of the construction of the bearing assembly by including additional components and operating structures within the construction of the shaft and the overall bearing assembly. Further, the additional structures are disposed on the stationary portion of the bearing assembly, i.e., the shaft, and are operable only to adjust the shape of the shaft to accommodate the deformation of the sleeve resulting from the heating of the sleeve during operation of the x-ray tube.
As a result, it is desirable to develop a structure and method for the formation of a bearing structure for an x-ray tube that can be formed with a simplified structure using low cost materials in a manner that significantly limits the formation of thermal gradients within the structure, thereby minimizing deformation of the bearing structure.